Viscous rotational speed control device

ABSTRACT

A rotational speed control device maintains a shaft rotation speed. The device includes a housing containing a viscous fluid and a shaft disposed in the housing and rotatable relative to the housing. A rotor is coupled with the shaft for rotation in the viscous fluid. In one arrangement, the rotor is axially displaceable along the shaft between a low-shear position and a high-shear position. A spring mechanism is disposed in the housing and biases the rotor toward the low-shear position. In another arrangement, the rotor may be designed to cooperate with the housing or other nonrotating features within the housing to vary a shear gap according to radially expanding components of the rotor. The rotor, housing and spring mechanism can be designed to cooperate to create large changes in braking torque in response to small changes in shaft rotational speed. This allows the rotation speed to be controlled within a relatively narrow range.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/103,168, filed Jan. 14, 2015, and is acontinuation of U.S. patent application Ser. No. 14/699,445, filed Apr.29, 2015, the entire content of each of which is herein incorporated byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

(Not Applicable)

BACKGROUND OF THE INVENTION

It is desirable to maintain a relatively constant speed of rotation ofan irrigation sprinkler regardless of nozzle size or pressure (i.e.,mass flow rate and fluid velocity). This invention results in a largeincrease in braking torque for a small increase in rotational speed andtherefore minimizes the change in rotational speed of the irrigationsprinkler when nozzle size or pressure is changed.

For many years, a braking system has been in use for sprinkler deflectorplates that utilizes a rotor that is immersed in a viscous fluid. Therotor is connected to a shaft which transmits the energy of the rotatingsprinkler or deflector plate into the viscous-braking mechanism. Therotor, viscous fluid and a portion of the shaft are contained within asealed housing. As the shaft and rotor rotate, the viscous fluid issheared between the rotor and the housing. As the shear rate increases,the braking torque that retards the shaft rotation also increases. As amatter of operation, the shear rate increases due to an increase in therotational speed of the shaft and therefore the surface speed of therotor. As a matter of design, the shear rate can be increased bydecreasing the gap between the rotor and the housing.

It is desirable to avoid requiring the shaft to move axially in and outof the housing. Axial movement can drag water and other contaminantsinto the seal and thereby cause water intrusion and/or excessive wear.Additionally, as the shaft moves into the housing, it pressurizes thefluid chamber, which can cause excess seal friction and seal wear unlessan expansion chamber is added.

BRIEF SUMMARY OF THE INVENTION

In some embodiments of this invention, the gap between the rotor and thehousing is changed automatically in response to changes in rotationalspeed of the shaft. The rotor is attached to the shaft in a manner thatcauses it to rotate with the shaft but allows it to move axiallyrelative to the shaft. The rotor is designed with an impeller-likefeature to create an axial force when it is rotated in the viscousfluid. The magnitude of the axial force is proportional to therotational speed of the rotor. A spring mechanism is located within thehousing in such a way as to resist the axial force of the rotor. Theaxial force of the rotor compresses the spring mechanism until thespring force matches the axial force being generated by the rotation ofthe shaft and rotor. These balancing forces are used to determine theaxial position of the rotor within the housing.

When the rotational speed of the input shaft changes due to changingpressures or nozzle size of the sprinkler, the rotor moves to a newaxial position. The rotor is designed to cooperate with the housing orother nonrotating features within the housing to vary the shear gap inresponse to the axial position of the rotor. The rotor, housing andspring mechanism can be designed to cooperate to create large changes inbraking torque in response to small changes in rotational speed of theinput shaft. This allows the sprinkler or deflector plate rotation speedto be controlled within a relatively narrow range.

Alternatively, the structure may be configured to use vanes to createradial movement rather than axial movement. Other alternate embodimentsuse mating threads working against either compression or torsionalsprings to create axial movement. Another alternate embodiment usesmechanical friction in addition to viscous fluid shear to create thebraking torque.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages will be described in detail withreference to the accompanying drawings, in which:

FIG. 1 shows an exemplary rotational speed control device in alow-braking stage;

FIG. 2 shows the rotational speed control device of FIG. 1 transitioningbetween the low-braking stage and the high-braking stage;

FIG. 3 shows the rotor;

FIG. 4 shows an alternative embodiment of the rotational speed controldevice with a modified rotor;

FIG. 5 shows the embodiment of FIG. 4 in an intermediate-brakingposition;

FIG. 6 shows the rotor for the embodiment of FIGS. 4 and 5;

FIG. 7 is a sectional view of an alternative embodiment;

FIG. 8 is a sectional view of the FIG. 7 embodiment with the rotortransitioning from a low-braking position to a high-braking position;

FIG. 9 is a sectional view of an alternative embodiment utilizing adisk-shaped rotor;

FIG. 10 is a sectional view of the embodiment in FIG. 9 in ahigh-braking position;

FIG. 11 is a sectional view of an alternative embodiment showing a3-stage device with added sheer area in the minimum braking position;

FIG. 12 is a sectional view of the FIG. 11 embodiment in a maximumbraking position;

FIG. 13 is a sectional view of the FIG. 11 embodiment including a drivecontrol band;

FIG. 14 is a graph showing typical compensating and noncompensatingviscous speed control performance;

FIGS. 15 and 16 show the rotational speed control device as part of anindustrial sprinkler;

FIG. 17 is a sectional view of an alternative embodiment utilizingnested cylinders;

FIG. 18 shows the shaft and a threaded hub for the FIG. 17 embodiment;

FIG. 19 is a detailed view of the rotor for the FIG. 17 embodiment;

FIG. 20 shows the rotor in the FIG. 17 embodiment approaching a maximumtorque position;

FIG. 21 is an upper perspective view of the brake assembly of analternative embodiment;

FIG. 22 is a vertical section view of FIG. 21 showing the hub and brakeshoes;

FIGS. 23 and 24 show a lower view of the FIG. 21 assembly with thehousing removed;

FIG. 25 is an upper perspective view of an alternative brake assembly;

FIGS. 26 and 27 are sectional views of the FIG. 25 embodiment;

FIG. 28 is a perspective view of the rotor in the FIG. 25 embodiment;

FIG. 29 is a side view of an alternative embodiment installed in anexemplary rotator sprinkler;

FIG. 30 is a sectional view of FIG. 29;

FIG. 31 is a sectional view of the brake assembly in the FIG. 29embodiment;

FIG. 32 is a sectional view of the FIG. 29 embodiment with the rotor ina high-braking position;

FIG. 33 is a perspective view of the housing in the FIG. 29 embodiment;

FIGS. 34-36 are perspective views of the rotor in the FIG. 29embodiment;

FIG. 37 is a sectional view of an alternative embodiment in an at-restposition and low-speed position;

FIG. 38 is a sectional view of the FIG. 37 embodiment in a high-brakingposition;

FIG. 39 is a perspective view of the threaded hub and torsion spring inthe FIG. 37 embodiment;

FIGS. 40 and 41 show the rotor of the FIG. 37 embodiment; and

FIG. 42 is a perspective view of the torsion spring in the FIG. 37embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The figures show several embodiments of a viscous rotational speedcontrol device 10. With reference to FIG. 1, a rotor 12 is rotatablewith a shaft 14 in a housing 16. The deflector plate 15 is secured tothe shaft 14 for rotation with the shaft 14. (See, e.g., FIG. 15.) Thehousing 16 is filled with a viscous fluid such as high-viscositysilicone fluid or the like. The housing 16 is closed at a bottom end andincludes a recess or channel 18 for receiving the shaft 14. A seal 20secured with a seal retainer 22 contains the viscous fluid within thehousing 16.

A retaining ring 30 and a bearing retainer 28 are used to axially locatethe ball bearing on the shaft 14. A lower bearing support 26 and anupper bearing support 24 cooperate to axially and radially locate theshaft bearing assembly in the housing 16. FIGS. 1-10 and 13 utilize aball bearing to support the axial and radial load that the water impartson the shaft 14. The axial load is transmitted to the housing 16 via thelower bearing support 26.

The rotor 12 includes a braking section 32 and an impeller 34. Theclearance between the braking section 32 and an inner wall of thehousing 16 is directly related to the amount of braking. In section Ashown in FIG. 1, the clearance between the braking section 32 of therotor and the inner wall of the housing 16 is relatively large forlow-braking. The inner wall of the housing 16 includes a step 36 thatnarrows the clearance between the braking section 32 of the rotor 12 andthe inner wall of the housing 16. The smaller clearance provides forgreater braking.

A spring mechanism such as a balancing spring 38 acts on the rotor 12and urges the rotor 12 toward the high-clearance, low-braking positionshown in FIG. 1. The balancing spring is secured via a spring retainer40. As the shaft 14 and rotor 12 are rotated, the impeller 34 drives therotor axially against the force of the spring 38. The clearance betweenthe impeller 34 and the inside wall of the housing 16 is relativelysmall to enable the impeller to more effectively drive the axialposition of the rotor 12. The axial force of the rotor 12 compresses thespring 38 until the spring force matches the axial force being generatedby the rotation of the shaft 14 and rotor 12. When the rotational speedof the input shaft 14 changes due to changing pressures or nozzle sizeof the sprinkler, the rotor 12 moves to a new axial position. The springmechanism may comprise any suitable structure for providing the bias,for example, opposing magnets or equivalent structure.

FIG. 2 shows the rotor 12 displaced axially by an increase in therotationally developed axial force. The rotor 12 is displaced such thatat least a portion of the braking section 32 is disposed adjacent thesmall clearance section B of the inner wall of the housing 16. By virtueof the smaller clearance, a greater braking resistance is provided. FIG.3 is an isolated view of the rotor 12.

FIGS. 4 and 5 show an alternative embodiment. In this embodiment, therotor 112 is generally cylindrical and is provided with screw threadslots 134 that define the impeller for driving an axial position of therotor 112. The inner wall of the housing 16 includes the step 36 suchthat with slower rotation, a larger portion of the rotor 112 ispositioned adjacent the large clearance section A of the housing forlower braking. In the smaller clearance section B, the screwimpeller/screw thread slots 134 can more effectively drive, and thesmaller clearance creates relatively high-shear braking on the threadmajor diameter for higher braking at higher rotation speeds. Withreference to FIG. 5, as the screw impeller 134 drives deeper, more ofits major diameter is in the low-clearance section B, which creates moreshear area and therefore, more braking. In FIGS. 4 and 5, the screwthread has a relatively short pitch with a broad thread crest. Theshearing action that creates the braking is done primarily between thethread crest (major diameter) and the housing 16. FIGS. 4 and 5 show therotor having a variable pitch screw thread with the pitch gettinggreater at the top. This is to create progressively wider thread crestsand more area subject to the high shear that occurs in the smallerclearance section B. This style of rotor may have a variable pitch asshown or a constant pitch depending on the desired performance. FIG. 6is an isolated view of the rotor 112.

FIGS. 7 and 8 show an alternative design for varying the shear andbraking based on the axial position of the rotor 212. In thisembodiment, the braking portion 232 of the rotor 212 is partconical-shaped, and the low-clearance section B in the housing 16 iscorrespondingly conical-shaped. FIG. 7 shows the rotor 212 in thelow-shear/low-braking position, and FIG. 8 shows the rotor 212 at leastpartially in the high-shear/high-braking position. Note that the sheargap 213 gets smaller as the axial position of the rotor 212 is displacedagainst a force of the spring 38 by the impeller 34 due to increasedrotation speeds.

FIGS. 9 and 10 show yet another alternative embodiment where the housingincludes a shoulder 313, and the braking portion 332 of the rotor 312 isspaced from the shoulder 312 by a variable shear gap C. As the axialposition of the rotor 312 is driven by the impeller 34, the shear gap Cis reduced as shown in FIG. 10 for higher shear and higher braking.

FIGS. 11-13 show an embodiment that not only changes the shear gap, butalso adds additional shear area. The rotor 412 includes a circular slot413 that engages a standing rib or circular ridge 414 formed in thehousing 16. The ridge 414 provides added shear area when engaged by therotor 412. In FIG. 13, the ridge 414′ forms parts of a drive controlband 420 inserted between the housing 16 and the rotor 412. The drivecontrol band 420 keeps the length of the screw portion/impeller 34 ofthe rotor 412 that is engaged in the tight diameter constant and givesbetter control of the rotor response.

FIG. 14 is a typical performance graph that illustrates the performancedifference between a conventional viscous brake and this device.

FIGS. 15 and 16 show the device 10 as part of a sprinkler. Note that thedevice 10 may also be employed in other forms of sprinklers includingones that would transmit torque to the device shaft via a gear train.

FIG. 17 shows another alternative configuration where the housing 16 isprovided with a plurality of circular grooves 514 separated by acylindrical ridge 515. The rotor 512 includes separated cylinders 516that are cooperable with the grooves 514 to increase or decrease theshear and braking based on an axial position of the rotor 512. Athreaded hub 522 is press fit to the shaft 14 so that it will rotatewith the shaft. As it begins to rotate, the rotor 512 also rotates withthe threaded hub 522, until such time that the rotation speed becomeshigh enough that the viscous shear overcomes the compression spring 538.FIG. 18 is an isolated view of the threaded hub 522 and the shaft 14.FIG. 19 is an isolated view of the rotor 512. FIG. 20 shows the rotor512 displaced axially from the position shown in FIG. 17 and approachingthe maximum torque position.

FIGS. 21-28 show two further embodiments for viscous fluid compensatingbrakes. Like prior embodiments, both units are filled with ahigh-viscosity silicone fluid or the like. Both designs utilize a shaftthat turns components that have radially expanding members. The radiallyexpanding members expand in response to rotation speed to increase thebraking torque by decreasing the viscous fluid shear gap. In theembodiment of FIGS. 21-24, the device is shown with a smooth outsidediameter on the brake shoes 614 that interacts with a smooth inside wallor inside diameter on the housing 16. The embodiment shown in FIGS.25-28 utilizes labyrinth-type geometry in the area of interaction, butsmooth or labyrinth could be used with both concepts.

FIG. 21 is an upper perspective view of the brake assembly alone. FIG.22 is a vertical section view of FIG. 21 showing the hub 612 and brakeshoes 614. FIGS. 23 and 24 show a lower view of the assembly with thehousing removed. The brake shoes 614 are in the minimum torque position,being biased there by integral springs 616. The hub 612 is secured tothe shaft 14 for rotation with the shaft 14. The hub 612 includes pivotposts 618 about which the brake shoes 614 are pivotable, respectively.The brake shoes 614 include exterior surfaces 620 that are curvedsubstantially corresponding to the inside wall of the housing 16. As thebrake shoes 614 pivot on the pivot posts 618, the curved exteriorsurfaces 620 of the brake shoes 614 are displaced radially outwardtoward the inside wall of the housing 16. An amount ofpivot/displacement of the brake shoes 614 varies according to a rotationspeed of the shaft 14.

The minimum torque position shown is the position of the shoes 614 whenthe unit is at rest or when turning very slowly. The leading edges 622of the shoes are shaped such that as rotation speed increases, the shoes614 will pivot outward against the force of the spring 616 to decreasethe fluid shear gap on the outside of the shoes 614, thereby increasingthe braking torque. That is, the brake shoes 614 are pivoted radiallyoutward by rotation of the shaft 14 by an amount that varies accordingto the rotation speed of the shaft 14. In one arrangement, the leadingedges 622 of the brake shoes 614 are tapered, although other shapes maybe suitable. The shoes 614 may be configured to press against the insidewall of the housing 16 to add a mechanical friction component to thebraking torque.

FIG. 25 is an upper perspective view of an alternative brake assemblyalone. FIGS. 26 and 27 are vertical section views. The housing 16 may beprovided with labyrinth rings 720 on the inside wall. The rings 720 maybe machined or molded with the housing 16, or the rings 720 may be inthe form of an insert 722 secured in the housing 16. As shown in FIG.26, a seal 724 such as an O-ring or the like seals a joint between thehousing 16 and the insert 722.

In this embodiment, the radially expanding member may include labyrinthsegments 726 that are displaceable between a contracted position (FIG.26) and an expanded position (FIG. 27). The segments 726 are sized andpositioned to interact with the labyrinth rings 720 in the expandedposition. The radially expanding member also includes a rotor section728 that is fixed to the shaft 14 and segment arms 730 coupled with therotor section 728. As shown, the labyrinth segments 726 are disposed onoutside surfaces of the segment arms 730. A spring section 732 actsbetween the rotor section 728 and segment arms 730 and biases thesegment arms 730 toward the contracted position. In one arrangement, therotor section, the spring section and the segment arms may be integralwith one another.

In FIG. 26, the rotor 712 is in the minimum torque position (at rest orat low speed), and in FIG. 27, the rotor 712 is nearing the maximumtorque position. The rotor 712 includes a plurality of angled propellerblades 718 at the top of the rotor 712 coupled with each of the segmentarms 730. As speed increases from the configuration in FIG. 26, thepropeller blades 718 force the rotor segments outwardly, and thelabyrinth segments 726 on the rotor 712 are interacting with thelabyrinth rings 720 in the housing 16 to decrease the fluid shear gapand thereby increase the braking torque. That is, the propeller blades718 are shaped and positioned to deflect the segment arms 730 against aforce of the spring section 732 toward the expanded position by rotationof the shaft 14 by an amount that varies according to a rotation speedof the shaft 14.

FIG. 28 is an upper perspective view of the rotor 712 alone. The rotor712 as shown is a single piece that may be molded out of a resilientplastic. The rotor could also be constructed as a multi-piece assemblyif desired.

FIGS. 29-36 show yet another alternative configuration of the brakeassembly. In principle, it is similar to the other describedembodiments. Axial motion, however, is powered with propeller-typeblades rather than a screw thread, and multiple concentric rings rotatein close proximity to multiple stationary rings to create theviscous-braking action.

FIG. 29 is a side view of the device 800 installed in an exemplaryrotator sprinkler. Typically, sprinklers operate over a wide range ofnozzle sizes and line pressure. FIG. 30 is a vertical cross section ofFIG. 29. FIG. 31 shows the brake assembly 800 including the housing 16,shaft 14 and rotor 812. As in previous embodiments, the housing 16 isfilled with a high-viscosity silicone fluid. In FIG. 31, the brake is atrest or turning very slowly, and the rotor 812 is in the raised orlow-shear position, being biased to that position by the compressionspring 38. In the position shown in FIG. 31, the braking torque is atits minimum due to the relatively large clearances between the rotatingand stationary members. As a torque is applied to the shaft 14, therotor 812 turns faster, which causes outside propeller blades 818 (andto a lesser extent, the blades connecting the rotor rings) to push therotor 812 down against the force of the spring 38. FIG. 32 shows therotor 812 in its lowermost position, which creates maximum braking bycreating large areas with minimal fluid gap. Depending on the torqueapplied to the shaft 14, the rotor 812 will float vertically between theminimum and maximum positions, finding an axial equilibrium between thespring load and the propeller loads.

FIG. 33 is a perspective view of the housing 16 showing openings 820 cutin the inner rings to allow fluid to move from one side of the rotor tothe other as the rotor moves axially. FIGS. 34-36 are variousperspective views of the rotor 812 showing the propeller blades 818. Asshown in FIGS. 31 and 32, the hub 822 is preferably machined out ofbrass square stock and press fit onto the shaft to enable transmissionof torque to the rotor while also allowing axial movement of the rotor.The hub 822 also provides support for the compression spring 38.

The exemplary sprinklers of FIGS. 29 and 30 are shown in the positionused when water is supplied through a drop tube, but the same sprinkleris often used in the inverted position, for example, on top of a centerpivot irrigation machine.

FIGS. 37-42 show yet another alternative embodiment. FIG. 37 is a crosssection of the assembly 900 when it is in the at-rest position andlow-speed position. The unit is filled with a viscous fluid. The rotor912 is biased to the up position by a torsion spring 938. The threadedhub 922 is press fit to the shaft 14 so that it will rotate with theshaft. As it begins to rotate, the rotor 912 also rotates with thethreaded hub 922, until such time that the rotation speed becomes highenough that the viscous shear between the rotor 912 and the housing 16gets high enough to overcome the torsion spring 938 and the viscousshear between the rotor 912 and the threaded hub 922 to rotate the rotorrelative to the threaded hub. This drives the rotor 912 down toward theposition shown in FIG. 38. In this position, the lower end of the rotoris contacting the housing, creating mechanical frictional torque inaddition to the viscous shear torque. FIG. 39 is a detailed view of thethreaded hub 922 and the torsion spring 938. FIGS. 40 and 41 aredifferent views of the rotor 912, and FIG. 42 is a detailed view of thetorsion spring 938.

The balancing springs in the various embodiments can be replaced withopposingly oriented magnets to generate the balancing force.

It should be noted that the operation of this device relies on therelative motion between the rotor and the housing. Therefore, it shouldbe recognized that the device could be designed to have the housingrotate about a nonrotating shaft and rotor rather than the describedexemplary embodiments where the shaft and rotor rotate within astationary housing.

The brake assembly can be mounted in various arrangements including oneswhere the shaft can be driven by a gear, rather than directly by adeflector plate, such as when used in a Big Gun Rotator™.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

The invention claimed is:
 1. A rotational speed control devicecomprising: a housing containing a viscous fluid; a shaft disposed inthe housing and rotatable relative to the housing; a radially expandingmember coupled with the shaft for rotation with the shaft relative tothe housing in the viscous fluid, the radially expanding member beingdisplaceable between a contracted position and an expanded position,wherein the radially expanding member is biased toward the contractedposition; and labyrinth rings disposed on an inside wall of the housing,wherein the radially expanding member comprises labyrinth segments thatinteract with the labyrinth rings in the expanded position.
 2. Arotational speed control device according to claim 1, wherein theradially expanding member further comprises a rotor section fixed to theshaft and segment arms coupled with the rotor section, wherein thelabyrinth segments are disposed on an outside surface of the segmentarms.
 3. A rotational speed control device according to claim 2, whereinthe radially expanding member further comprises a spring section betweenthe rotor section and the segment arms, the spring section biasing theradially expanding member toward the contracted position.
 4. Arotational speed control device according to claim 3, wherein theradially expanding member further comprises a propeller blade coupledwith each of the segment arms.
 5. A rotational speed control deviceaccording to claim 4, wherein the propeller blades are shaped andpositioned to deflect the segment arms against a force of the springsection toward the expanded position by rotation of the shaft by anamount that varies according to a rotation speed of the shaft.
 6. Arotational speed control device according to claim 3, wherein the rotorsection, the spring section and the segment arms are integral with oneanother.
 7. A rotational speed control device according to claim 1,wherein the radially expanding member is biased toward the contractedposition by a spring.
 8. A rotational speed control device according toclaim 7, wherein the spring is integral with the radially expandingmember.
 9. A rotational speed control device according to claim 1,wherein the radially expanding member is configured such that theradially expanding member is spaced a first distance from an inside wallof the housing in the contracted position and a second distance from theinside wall of the housing in the expanded position, and wherein thesecond distance is smaller than the first distance.
 10. A rotationalspeed control device comprising: a housing containing a viscous fluid; ashaft disposed in the housing and rotatable relative to the housing; anda radially expanding member coupled with the shaft for rotation with theshaft relative to the housing in the viscous fluid, the radiallyexpanding member being displaceable between a contracted position and anexpanded position, wherein the radially expanding member is biasedtoward the contracted position, wherein the radially expanding membercomprises: a hub secured to the shaft; at least one brake shoe coupledwith the hub, wherein the brake shoe is radially displaceable in thehousing relative to the hub; and a spring connecting the brake shoe tothe hub.
 11. A rotational speed control device according to claim 10,wherein the radially expanding member comprises two brake shoes.
 12. Arotational speed control device according to claim 11, wherein the brakeshoes comprise exterior surfaces that are curved substantiallycorresponding to an inside wall of the housing, and wherein the brakeshoes are pivotable relative to the hub via the spring connecting eachof the brake shoes to the hub.
 13. A rotational speed control deviceaccording to claim 12, wherein leading edges of the brake shoes areshaped such that the brake shoes are pivoted radially outward byrotation of the shaft by an amount that varies according to a rotationspeed of the shaft.
 14. A rotational speed control device according toclaim 13, wherein the leading edges of the brake shoes are tapered. 15.A rotational speed control device according to claim 10, wherein thespring is integral with the hub and the brake shoe.
 16. A rotationalspeed control device according to claim 10, wherein a leading edge ofthe brake shoe is shaped such that the brake shoe is pivoted radiallyoutward by rotation of the shaft by an amount that varies according to arotation speed of the shaft.